1. Field of the Invention
The present invention relates to a power output apparatus and a method of controlling the same. More concretely, the present invention pertains to a power output apparatus for outputting power generated by an engine to a drive shaft with a high efficiency and a method of controlling such a power output apparatus.
2. Description of the Prior Art
In known power output apparatuses mounted on a vehicle, an output shaft of an engine is electromagnetically linked with a drive shaft, which connects with a rotor of a motor, by means of an electromagnetic coupling, so that power of the engine is transmitted to the drive shaft (as disclosed in, for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814). In this prior art power output apparatus, the motor starts driving the vehicle. When the revolving speed of the motor reaches a predetermined level, an exciting current is given to the electromagnetic coupling to crank the engine, simultaneously with a supply of fuel to the engine and ignition with a spark, so that the engine starts its operation. After the start of the engine, the power output from the engine is transmitted to the drive shaft via the electromagnetic coupling to run the vehicle. When the power transmitted to the drive shaft via the electromagnetic coupling does not satisfy the required power of the drive shaft, the motor is driven to supplement the insufficiency. While transmitting the power to the drive shaft, the electromagnetic coupling regenerates electric power corresponding to a slip of its electromagnetic linkage. The regenerated electric power is stored in a battery to be used for starting the vehicle, or is used as power of the motor to supplement the insufficient power of the drive shaft.
This power output apparatus requires a motor having a large output torque. Only the power from the motor is used to start or launch the vehicle, and the motor with a large output torque is accordingly required for sufficient acceleration. Taking into account the starting operation on a rising slope, the motor is required to have a greater output torque. The motor having a large output torque is generally rather bulky and undesirably increases the size of the whole power output apparatus. This is especially not suitable when the power output apparatus is mounted on a limited space, such as a vehicle. The battery that supplies electric power to the motor is also required to have a large capacity and thereby has a relatively large size.
The applicant has invented a technique (not filed) of enabling the power output from an engine which starts its operation before a start of the vehicle to be transmitted to a drive shaft via an electromagnetic coupling, as well as addition of the power from a motor to the drive shaft. This lowers the required output torque of the motor and makes the whole power output apparatus relatively compact. At the moment of starting the vehicle, the revolving speed of the drive shaft is extremely small, so that the electric power regenerated by the electromagnetic coupling becomes greater than the electric power consumed by the motor. This may result in excess charging and damage the battery.